This invention relates generally to apparatus for mixing and dispensing bone cement. In particular, this invention relates to improvements to a disposable preassembled kit including a collection of preparation apparatus used to mix and dispense bone cement. The improvements are directed toward eliminating entrained air in the mixed cement and improving the mixing and dispensing characteristics.
In many orthopaedic surgical procedures, bone cements are used to fix implants to the bone. Conventional bone cements are generally polymeric materials which are prepared by copolymerization of the components as needed. Bone cement is prepared by copolymerizing a liquid monomer and a powdered copolymer, such as methyl methacrylate and polymethyl methacrylate or methyl methacrylate-styrene. During mixing of the constituent components of the cement, air bubbles may be formed within the cement. It is thought that to improve the resultant strength of the cement, the air bubbles must be evacuated from the mixture to ensure a uniform reaction product. Consequently, the mixing of the constituent components is ideally performed in a vacuum.
Mixing separate constituent components within a mixing cartridge is well-known in the art. For example, U.S. Pat. Nos. 5,624,184 and 5,586,821, which are assigned to the assignee of the present invention and are hereby incorporated by reference, disclose a mixing cartridge comprising two telescoping tubular cylinders. The telescoping cylinders are positionable into a fully extended position in which the bone cement components are mixed under vacuum, a vacuum release position in which the inner cylinder is partially retracted within the outer cylinder, and a fully retracted position in which the cartridge is adapted for installation into a bone cement mixing gun. A cap is installed on the proximal end of the outer cylinder and includes an opening therethrough which receives the shaft of an agitator. The cap also includes a port for connection thereto of a vacuum pump which is used to draw a vacuum in the cartridge when in the fully extended position. The portion of the agitator shaft which extends into the cartridge includes agitator paddles extending therefrom for mixing the bone cement. The other end of the agitator shaft which extends from the opening in the cap includes a handle thereon for reciprocating movement of the agitator within the cartridge. The shaft includes a tubular outer sleeve and an inner rod axially disposed therein which terminates in an integral dumbbell-shaped end plug. The shaft sleeve is frangible and has a detachable distal end, so that after mixing is completed, the shaft can be broken away from the cap along with the inner rod. This results in the opening in the cap retaining the distal end of the shaft having the agitator paddles integrally connected thereto. A suitable cement nozzle can then be affixed to the opening (outlet port), and the cartridge inserted into an applicator gun. The inner cylinder includes a piston plug which is axially movable toward the cap end of the cartridge by operating the cement gun.
In use, the above described cartridge is initially fully extended and the bone cement constituents are placed therein. Thereupon, the cap is installed on the outer cylinder and a vacuum pump is attached to the outlet port contained on the cap. A vacuum is drawn on the cartridge by reciprocating the handle on the vacuum pump. The bone cement components are then mixed under a partial vacuum by reciprocating the agitator handle. After mixing is completed, the inner cylinder is partially retracted within the outer cylinder which releases the hermetic seal therebetween and thereby causes the vacuum to be released. The cartridge is then positioned into its fully retracted position and installed into an applicator gun for injecting bone cement into a patient.
It is desirable to prevent air from becoming entrained within the bone cement. It is also desirable to further improve the mixing characteristics of the above-described kit.